A direct conversion receiver typically includes a mixer, a baseband filter, a baseband variable amplifier, RF variable amplifiers and an analog-to digital-converter. The main difference between a direct conversion receiver and a heterodyne receiver is that a direct conversion receiver outputs a signal centered around DC rather than around an intermediate frequency.
FIG. 1 is a block diagram of a typical direct conversion receiver. G1 represents the gain of first RF amplifier 10. G2 represents the gain of the second RF amplifier 12. Mixer 14 provides frequency translation. Low pass filter 16 provides frequency selectivity. G3 represents the gain of the baseband amplifier 18. Analog-to-digital converter 20 converts the analog signal it receives to a digital signal.
The finite isolation (i.e., physical separation) between signal lines S1 and L1 causes a certain amount of LO leakage in mixer 14 and which is mixed with the down-converted baseband signal to appear at the output of mixer 14, a phenomenon commonly referred to as self-mixing, as shown in FIG. 2. Thus signal S3 supplied by mixer 14 is a composite of the down-converted received waveform, DC component, and high frequency components. Such high frequency components are filtered out by filter 16.
The DC component of signal S3 may vary depending on the gain G2 of amplifier 12. Low-pass filter 16 may also introduce gain dependent DC offsets. Since ADC 20 can only handle a certain finite voltage swing, the potentially large DC offsets coming from both the RF and the baseband portion of the receiver may saturate ADC 20. Therefore, the DC offsets must be minimized before reaching the ADC 20 in order to preserve the integrity and dynamic range of the signal entering the baseband demodulator. The receiver often includes an analog front end and a baseband demodulator, as is well known. This may be achieved using conventional calibration techniques by injecting a correction factor in a signal line, such as signal line S3, to cancel out the DC offset. This process is often referred to as DC calibration or DC offset correction.
However, as gains G2 and G3 vary, the desired correction factor in general also varies. This will cause perturbations in the signal unless the signal path rapidly adjusts the correction factor as a function of these gains. This can be accomplished by using look-up tables (LUTs) which store the correction factors corresponding to different gain settings a priori.
In typical applications it is desirable to cancel DC offsets prior to the analog-to-digital-converter. In typical receivers, such as those used in television reception, two known techniques are used to handle DC offset. In accordance with the first conventional technique, shown in FIG. 3, the DC offset estimation is performed for all possible gain partitioning between G2 and G3, and the results are then stored in a look-up table (LUT) 22. LUT 22 is referenced every time the receiver changes either G2 or G3. The number of entries in LUT 22 is thus (G2×G3). Although relatively straight-forward, this technique is slow, especially as the gain ranges for G2 and G3 become large. If the DC offset estimate values are stored in hardware to improve the speed of operation, the semiconductor die size increases, thus increasing the cost.
Referring to FIG. 4, in accordance with a second conventional technique, DC offset estimation is performed for a fixed number of G2, G3 pairs, and stored in table 24. This technique reduces complexity in hardware but requires that the demodulator perform dynamic DC offset cancellation. Since G2 and G3 vary depending on channel conditions, it is difficult to select the right pair of G2 and G3 values that will keep the DC offset swing to a minimum value. This technique also requires the DC offsets value not to vary much with the gain.